The invention relates to a method of producing tantalum (V) nitride by nitridation of an oxidic tantalum (V) compound. The invention further relates to the use of the tantalum (V) nitride obtained in accordance with the invention as a coloring pigment.
Coloring bodies, such as pigments, are subject to many adverse influences, depending on the type of application and the use of the articles colored with them. In the case of plastic articles colored with pigments based on oxidic, sulfidic or selenidic heavy-metal compounds, acidic or alkaline solutions of toxicologically hazardous components also can be released, e.g., Ni, Co, Zn and Cr from spinels and Cd from cadmium sulfide yellow, as well as Cd and Se from cadmium sulfoselenide red.
A further problem is the release of toxic heavy metals during combustion in garbage incinerating plants of discarded plastics colored in this manner. Therefore, there is a particular interest in finding pigments in the yellow to red spectral range which contain fewer hazardous components than, e.g., CdS yellow and Cd (S, Se) red.
The nitridation of tantalum (V) compounds with ammonia at high temperatures was investigated in various manners. During this process, red tantalum (V) nitride (Ta.sub.3 N.sub.5) is formed via dark green, bright green, yellowish brown and reddish brown intermediate stages. However, applicants are not aware of any suggestions for the use of tantalum (V) nitride (Ta.sub.3 N.sub.5) as a coloring pigment.
The nitride of pentavalent tantalum can be produced according to H. Moureau and C. H. Hamblet (J. Amer. Chem. Soc. 59, 33-40 (1937)) by means of ammonolysis of TaCl.sub.5. This document is entirely incorporated herein by reference. The intermediate stage Ta(NH.sub.2).sub.2 Cl.sub.3 in the process described therein is decomposed in an ammonia flow at 650.degree. to 750.degree. C. The nitride obtained by this process was not pure because it contained oxidic components. In addition, very long reaction times were required; e.g., 6 days at 750.degree. C.
H. Funk and H. Bohland (Z. Anorg. Allg. Chem. 334, 155-62 (1964)) describe a process wherein the reaction times were shortened by using (NH.sub.4).sub.2 TaF.sub.6 as the initial product. Due to the volatility of TaF.sub.5 and NH.sub.4 F, the yields of Ta.sub.3 N.sub.5 are low and/or the resulting products contain fluorine. This document also is entirely incorporated herein by reference.
The Funk and Bohland document also disclosed the nitridation of tantalum (V) oxide (Ta.sub.2 O.sub.5) at 800.degree. C. with ammonia. In this process, oxide-containing nitride (Ta.sub.3 N.sub.5 (O)) forms at 800.degree. C., the color of which was not characterized in detail. The nitridation of pure tantalum pentoxide with purified ammonia was re-investigated by G. Brauer and J. R. Weidlein (Angew. Chem. 77, 218-9 (1965)) with reference made to the investigation of Funk and Bohland. The Brauer and Weidlein document also is entirely incorporated herein by reference. Brauer and Weidlein described that in the presence of titanium shavings as getter for oxygen and analogously for the decomposition of ammonia, the reaction requires a reaction time of 36 to 120 hours at 860.degree. C. to 920.degree. C. The reaction is completed only in the presence of the titanium shavings. However, while reworking the investigations of Brauer and Weidlein by the applicants, the reaction times were not able to be corroborated. The color formation was not terminated until after a reaction time of 150 hours, that is, no further color shift from reddish brown to red takes place.